1. Field of the Invention
The present invention relates to an electrostatic ink-jet printer for printing characters and images on a recording medium by using a liquid toner dispersed with electrically charged and colored particles and agglomerating the electrically charged and colored particles at an end of an jutting eject point under an electrostatic force and separating droplets of the liquid toner from the top of the jutting eject point by utilizing ultrasonic vibration.
2. Description of the Related Art
Recently, as an output device of a personal computer, there are widely used ink-jet type printers for printing characters and images on a recording medium such as a recording paper by ejecting ink particles toward an opposite surface of the recording medium.
Among them, an electrostatic ink-jet printer draws much attention, which uses a liquid toner dispersed with electrically charged and colored particles (referred to as electrified colored particles hereinafter) and ejecting the electrified colored particles on a surface of a recording paper by making use of an electrostatic force.
The reason for the attention is following. In such printing apparatuses as a bubble-jet-type printer which causes ink to eject from a nozzle by making use of thermal expansion of ink and a piezo-type printer which causes ink to eject by using a piezo-pump, they require not only a nozzle which causes a chock of ink passing therethrough but also require provision of various kinds of nozzles each having a different internal diameter for printing an image in different tone (gradation). On the other hand, the electrostatic ink-jet printer does not require any nozzles. In addition, a formation of the tone is easily realized by only changing a width or a height of a pulse of a voltage to be applied.
These electrostatic type printers are disclosed in the Japanese Patent Application 8-149253 and the Japanese Patent Publication 7-502218, wherein a liquid toner dispersed with electrified colored particles in an insulation liquid is supplied to an ejecting point of a pointed electrode body, and is ejected by a Coulomb force developed by applying a voltage having the same electric polarity as that of the electrified colored particles, resulting in printing on a recording paper.
In order to form an image, a bias voltage is always applied to an electrode to agglomerate the electrified colored particles to the ejecting point, and an ejection voltage is applied to the electrode responsive to an image input signal. This electrostatic printing method enables to realize a high density printing because the electrified colored particles are ejected after being agglomerated. In addition, the electrostatic printer has a simple structure, so that it is possible to form multi-ejecting points by aligning a plurality of electrodes in-line, resulting in a feature to allow a high speed recording.
Further, as another ink-jet printer, there is proposed a printer device utilizing an ultrasonic wave in a following English document:
[ACOUSTIC INK PRINTING: PRINTING BY ULTRASONIC INK EJECTION] (IS&T's English International Congress on Advances in Non-Impact Printing Technologies (1992), page 411-415).
In this electrostatic apparatus, an ultrasonic wave is generated from an ultrasonic generator submerged in ink. The generated ultrasonic wave is focused on a surface of ink by using an acoustic lens. Thereby, the ink is locally vibrated, so that minute ink droplets are separated from the ink and ejected.
Incidentally, important points to stably eject the liquid toner are how to provide the liquid toner to a surface of the ejecting point and how to effectively eject the electrified colored particles. Because a variation of a supplied quantity of the liquid toner containing the electrified colored particles brings about a variation of quantity of an ejected liquid toner, resulting in an density unevenness in the printed character and image.
As far as this is concerned, a satisfactory result has not been obtained in the abovementioned printers disclosed in the Japanese Patent Application 8-149253 and the Japanese Patent Publication 7-502218.
Further, in the printer utilizing the ultrasonic wave, ink droplets are ejected by a mechanical vibration. Thus, the colored particles need not be electrified. However, in order to eject the ink droplets from a flat liquid surface of ink, it is necessary to precisely focus the ultrasonic wave on the liquid surface of ink by using a high ultrasonic frequency of 150 MHz. Therefore, a height of the liquid surface has to be precisely controlled to a degree of an order of submicron. In addition, an acoustic lens system needs a high accuracy, resulting in a great difficulty in a realization of the printer.
Furthermore, in order to eject the ink droplets from the flat level of the liquid ink surface, it is difficult to use an ink having a high viscosity. Inevitably, a dye having a low viscosity has to be used as the ink, resulting in a drawback of easily oozing out (permeating) to the paper. In other words, in order to obtain a high print quality, a pigment having a high viscosity without an ink permeation can not be used here instead of the dye.